1. Field of the Invention
The present invention relates to a photoelectric encoder and, more particularly, to the improvement of a scale and an optical system.
2. Description of the Prior Art
Various kind of encoders are used for various measuring machine, tools and, recently, information apparatuses for the purpose of detecting the displacement between two members which relatively move. Especially, photoelectric encoders are used in various fields because they can detect displacement in a noncontact state.
A photoelectric encoder is composed of a grating provided on each of the two members which relatively move, and a light emitting element and a light receiving element for detecting the overlap of the two gratings.
As conventional photoelectric encoders, what is called a three-grating type photoelectric encoder (Journal of the Optical Society of America, 1965, vol. 55, No. 4, pp. 373 to 381) for detecting the displacement from a change in the overlap between three gratings, as shown in FIG. 7, and a reflection type photoelectric encoder (Japanese Patent Laid-Open No. 198814/1982) such as that shown in FIG. 8 are known as well as ordinary encoders for detecting the overlap of two gratings.
A three-grating type encoder 10 shown in FIG. 7 includes a grating 12 on the light emission side and a grating 14 on the detection side which are arranged in parallel with each other, a reference grating 16 which is inserted between the gratings 12, 16 in parallel therewith in such a manner as to be relatively movable, a light emitting element 18 disposed on the left-hand side of the grating 12 in the drawing, and a light receiving element 20 disposed on the right-hand side of the grating 14 in the drawing.
The light emitted from the light emitting element 18 reaches the light receiving element 20 through the grating 12 on the light emission side, the reference grating 16 and the grating 14 on the detection side. The projected light is restricted by each of the gratings 12, 14 and 16, and photoelectrically converted by the light receiving element 20. The converted signal is further amplified by a preamplifier 22 and produced as detection signal s.
When the reference grating 16 relatively moves to the grating 12 on the light emission side and the grating 14 on the detection side, for example, in the direction indicated by the arrow X, the quantity of light emitted from the light emitting element 18 and shielded by the gratings 12, 16 and 14 gradually varies, and the detection signal s is output in the form of substantially a sine wave.
The pitch P.sub.1 of the reference grating 16 corresponds to the wavelength P of the detection signal s, so that the amount of relative movement of the reference grating 16 is measured from the wavelength of the detection signal s and a divided value thereof.
It is therefore possible to detect the amount of relative movement of a main scale 24 and index scales 26 by disposing the reference grating on the main scale 24 and the grating 12 on the light emission side and the grating 14 on the detection side on the respective index scales 26.
FIG. 8 shows a reflection type photoelectric encoder 10. The same numerals are provided for the elements which correspond to those shown in FIG. 7 and explanation thereof will be omitted.
In this example, the main scale 24 and the index scale 26 are made of light transmissive glass, the main scale 24 being provided thereon with the reference grating 16 made of a reflective material and the index scale 26 being provided thereon with the light receiving elements 20 in the form of a grating.
In this way, slits corresponding to those formed between the grating 12 on the light emission side and the grating 14 on the detection side in FIG. 7 are formed on the index scale 26.
The light from the light emitting element 18 is adjusted to parallel rays by a collimator lens 28 and projected from the back surface of the index scale 26.
As a result, the light is transmitted only through the portion at which the light receiving element 20 is not formed and projected onto the main scale 24.
The light reflected by the reference grating 16 on the main scale 24 proceeds again toward the index scale 26 and it is photoelectrically converted by the light receiving elements 20.
On the other hand, the light projected onto the gaps of the reference grating 16 is transmitted through the main scale 24, which is made of glass, and does not reach the light receiving elements 20.
In this way, the amount of relative movement of the main scale 24 and the index scale 26 is detected by the light receiving elements 20 as a substantially sine wave in the same way as in the system shown in FIG. 7.
However, in the three-grating type transmission photoelectric encoder shown in FIG. 7, since it is necessary to dispose the light emitting element 18 and the light receiving element 20 on the outsides of the scales 24 and 26, the number of parts required is large, which makes the manufacture of apparatus complicated, and the total size becomes inconveniently large.
This is the same with the reflection type photoelectric encoder shown in FIG. 8. Especially, in this system, since it is necessary to provide the collimator lens 28 for distributing light, the apparatus inevitably has a large size.